I have a 1975 C3 that came factory with a THD 400 automatic transmission, 3:08 differential, and tire size P255 60R15.

I replaced this transmission with a 700R4.  By doing so, this affected my speedometer to where it reads 6 - 7 mph fast. Speedometer displays 60 mph, actual is 53-54 mph.

 I confirmed this by road side traffic radar units, and a cellular phone  android GPS Speedometer program.

The color of its speedometer gear is "red." 

Does anyone know what color speedometer gear is required for this 700R4 transmission to bring it closer to being accurate?

Thnx, James

I don't know what color is correct (my transmission is 4-speed). but. . .

I'd go by the GPS -- I've seldom seen one of those roadside displays even close to the correct speed.

Having the speedo read a bit high is probably not a bad thing, it may reduce getting some speeding tickets. . .

How do you know what the rear gearing is?  Is there a tag on the diff?

Thanks for your comments!!

It's just my preference to ensure all components on this old classic are in accurate and proper working condition.

The speedometer is a crucial  measuring device and a must have to ensure you are operating the motor vehicle safely within the limits of the absolute speed laws. 

Having the speedometer read 6 - 7 miles fast does give you a buffer, agreed, but I require all to function with accuracy.

The differential,  at least mine, has a flange located on the bottom of the case close to the yoke with the letters "AW" which indicates the differential ratio is a 3:08.  Check put the below chart:

Thanks for the cart!  I'll check it out next time I'm under the car.

255x60 tires did not come on 75 Vettes so you need to compare the diameter to a GR70x15 tire.

You are correct in your response. Already working on calculations.

Thanks for your response. James

If you still have the original 400, see what gear tooth counts it has, and get the same. You need to know BOTH speedo gear tooth counts, not just the driven gear. I do have a speedo gear ratio chart at the shop I can post to help determine what you need.

Joel Adams
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

The THD 400 long gone, thanks for your response.  JAMES

I'm an original owner of a 1975 with a L48 and M40 (Turbo Hyro 400) transmission. I have attached the page from the Factory Assembly Manual. I didn't see any red gears, but there is a pink gear. I hope this will help you. 

Note: The code on my transmission from my build sheet and on the transmission housing is CK and the axle ratio is 2.733 (GU2).

Ron

Going off of Ron's chart, the M40(400) with 3.08 rear would have a 17 tooth drive gear, with a 40 tooth driven gear. These are the tooth counts you'll need to get your speedo properly calibrated(factory). Both gears should be available. In order to use the 40t driven gear, you will need a speedo gear housing that will orient the gear in the proper manner to mesh with the driven gear. The housings have numbers on them that correspond to the tooth counts they will work with. IF your current housing has the numbers from 40 to 43, it will work. If it does not, then you will need to source the correct housing, along with the gears. 

You really need to count the teeth on your current drive and driven gears(you cannot always go by colors....count the teeth!). The current drive gear may possibly be a 17 tooth gear already, and if so, you will not need to change it. Changing the drive gear will require removing the tailhousing from the transmission. I'm thinking your "red" driven gear is 37 tooth. If your drive gear is 17t, then the 3 tooth difference between your current gear and what you need will be your 6-7mph error.

Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Summit Racing speedo gear calculator chart.  Might help.

https://static.summitracing.com/global/images/chartsguides/a/ado-3860345.pdf

Thnx for your comment, I replaced the THD400 with a 700R4.

You are correct,  37 teeth red gear. Thnx for your comment. 

The gear is  red and has 37 teeth.  Thnx for your comment. 

Dude, no one knows who you are talking to.  Click the "Quote" button and then add your comment.